Before

Before
 

-We could say 'Why do you ask' or 'Why did you ask'. Though the question was asked several seconds before/ago, the situation is still in front of your eyes, so the present tense can still be used.

Talking about language use in general situations, are BEFORE and AGO both correct in this case?

In many cases they can be used interchangeably, however they have separate uses.
Why do you ask is best used if the person who asks the question does so habitually. (as in why do you ask me what I eat for lunch all the time?

Why did you ask is a response to a non-habitual question. Example:
How much money do you make? Why did you ask me that?
Conversely you can say: why does he ask what people earn all the time?

Thank you, Poli

eg We could say 'Why do you ask' or 'Why did you ask'. Though the question was asked several seconds before/ago, the situation is still in front of your eyes, so the present tense can still be used.

Talking about language use in general situations, are BEFORE and AGO both correct in this case?

Both are correct.

Thank you~

eg Again he shouted, even louder than before, and again no whisper came back from the friends whom he had left such a short time ago.

Do you think nowadays BEFORE must be used instead? The usual distinction should be:

-I did something a month ago. (=a month back from now)
-I did something a month before. (=a month back from a certain time in the past)

Within the context of a story, the moment of the current action is the only "now" that matters, even though the action of the story is written using the simple past. Within this sentence "ago" is perfectly natural, as is "before", and they have the same meaning: a month earlier than the "now" of the story.

Thank you~